1. Field of the Invention
The present invention relates to a focus condition detecting device for cameras wherein the object image formed by the camera-objective lens is re-imaged as first and second images on first and second sensors through a pair of re-imaging lenses or relay lenses and the change of the relative positions of the first and second images is detected by the first and second sensors for detecting the focus condition of the camera-objective lens.
2. Description of the Prior Art
Many focus condition detecting devices for cameras of the type as mentioned above have been proposed, for example, in Japanese Laid-open Patent Application with the laid-open (Tokkaisho) No. 52-95221, and their typical optical system has constructions as shown in FIG. 1. In FIG. 1, a condensor lens 4 is disposed in the vicinity of the predetermined focal plane of camera-objective lens 2 which plane is optically equivalent to the film plane. At the back of the condensor lens 4 a pair of re-imaging lenses 6 and 8 are arranged symmetrically with respect to optical axis l (hereinafter referred to as main optical axis) of camera-objective lens 2 so that the image formed by the camera-objective lens on the predetermined focal plane F is re-imaged by means of the pair of re-imaging lenses 6 and 8. The reference characters A, B and C indicate respectively the front focus image, in-focus image and rear focus image formed by camera-objective lens 2.
Re-imaging lenses 6 and 8 form the first and second images of (A1) and (A2), (B1) and (B2), and (C1) and (C2) respectively corresponding to front focus image A, in-focus image B and rear focus image C. When image A, B and C are formed, for example, as the images shown with the upward directed arrow marks, the first and second images corresponding to these images are shown as the images shown by the downward directed arrow marks and the distance between the first and second images varies depending on the focus condition of camera-objective lens 2. Accordingly, if photocell arrays are arranged at or in the vicinity of the position conjugate with predetermined focal plane F with respect to condensor lens 4 and re-imaging lenses 6 and 8 and the change of the relative positions of the first and second images is detected from the outputs of those line photocell arrays, the focus condition of camera-objective lens 2 can be detected.
Note that numerals 10-1 and 10-2 are diaphragm aperture masks installed in front of re-imaging lenses 6 and 8.
In the focus-condition detecting device using the optical system shown in FIG. 1, it is important for the detection of the focus condition of camera objective lens with high accuracty, that the first image coincides with the second image in regard to the luminous level and luminous distribution, and that the distance between the first and second images will not vary depending on the image height, i.e., the position of each point of a target object with respect to main optical axis l of camera-objective lens 2.
In the conventional focus condition detecting device using the optical system shown in FIG. 1, however, a pair of biconvex lenses are used as the re-imaging lenses. Since such biconvex lenses are normally molded by different molds and an eccentricity is liable to occur on both spherical surfaces of the lenses, resulting in such disadvantages as an inconsistency in luminous distribution caused by the coma and/or distortion in the first and second images formed by these lenses, and change in the distance between the both images depending on the image height.
Also, when the bicovex lens is used as the re-imaging lenses, the distance between the front principal point of the biconvex lens and a diaphragm aperture mask must be made short, resulting in a substantial reduction in the luminous intensity of the first and second images at the positions distant from the main optical axis, on the first and second line sensors. This will enhance the tendency that the symmetry of the illumination distribution in both images is impaired.
Consequently, if biconvex lenses are used as a pair of re-imaging lenses, a high-precision in the detection of the focus condition of the camera-objective lens can not be expected.